The choice is yours but if you sell the buyer will wish to make a profit so you will not get the full value. Is your endowment a 'with profits' type where a sum is added to the surrender value each year, or is it unit linked?
If unit linked the value will fluctuate and is a bit of a gamble. On maturity a further bonus will probably be added. One of my policies that recently matured almost doubled in value when the final bonus was added although the annual bonuses had been very poor.. As you have made other arrangemtents to repay your mortgage I would think it best with just 8 years left to continue to pay the premiums and wait for the policy to mature.